ABSTRACT
While magnesium is astronomically observed in small molecules, it largely serves as a contributor to silicate grains, though how these grains form is not well-understood. The smallest hypermagnesium oxide compounds (Mg2O/Mg2O+) may play a role in silicate formation, but little vibrational reference data exist. As such, anharmonic spectroscopic data are computed for X 1Σ+g Mg2O, ã 3Σ+u Mg2O, and X2Σ+g Mg2O+ using quartic force fields (QFFs). Explicitly-correlated coupled-cluster QFFs for the neutral species perform well, implying that full multireference treatment may not be necessary for such systems if enough electron correlation is included. Equation-of-motion ionization potential (EOMIP) methods for X2Σ+g Mg2O+ QFFs circumvent previous symmetry breaking issues even in explicitly-correlated coupled-cluster results, motivating the need for EOMIP treatments at minimum for such systems. All three species are found to have high-intensity vibrational frequencies. Even so, the highly intense fundamental (X 1Σ+g Mg2O: 894.7 cm-1/11.18 µm; ã 3Σ+u Mg2O: 915.0 cm-1/10.91 µm) for either neutral state may be astronomically obscured by the polycyclic aromatic hydrocarbon 11.2 µm band. Mg2O+ may be less susceptible to such obfuscation, and its ν1 intensity is computed to be a massive 4793 km mol-1.
ABSTRACT
The addition of sp-carbon-containing molecules to polycyclic sp3 tetrahedrane (c-C4H4) results in the formation of both o-benzyne (c-C6H4) and benzene (c-C6H6). Since both c-C6H4 and c-C6H6 have been detected in the interstellar medium (ISM), providing additional pathways for their possible astrochemical formation mechanisms can lead to the discovery of other molecules, such as c-C4H4, benzvalyne, and vinylidene (:CCH2). Addition of diatomic carbon (C2), the ethynyl radical (C2H), vinylidene, and acetylene (HCîCH) to c-C4H4 is undertaken in individual pathways through high-level quantum chemical computations at the CCSD(T)-F12b/cc-pVTZ-F12 level of theory. The resulting C2 addition pathway proceeds barrierlessly through benzvalyne as an intermediate and reaches a true minimum at c-C6H4, but no leaving groups are produced which is required to dissipate excess energy within an interstellar chemical scheme. Similarly, the C2H addition to c-C4H4 produces benzvalyne as well as its related isomers. This pathway allows for the loss of a hydrogen leaving group to dissipate the resulting energy. Lastly, the HCîCH and :CCH2 addition pathways follow through both benzvalene and benzvalyne in order to reach c-C6H6 (benzene) and c-C6H4 (o-benzyne) as well as H2 as the required leaving group. Although there is a barrier to the HCîCH addition, the :CCH2 addition presents the contrary with only submerged barriers. These proposed mechanisms provide alternative possibilities for the formation of complex organic molecules in space.
